Flat roofs

Why do seams fail on a flat roof?

Short answer: Seams can fail through poor preparation, contamination, insufficient bonding or welding, movement, incompatible materials and water sitting at defective details.

What this means in practice

Flat-roof performance depends on the whole build-up: structural deck, vapour control, insulation, falls, waterproofing, outlets, kerbs and edge details. Choosing a branded membrane while ignoring trapped moisture or poor drainage simply locks the original fault beneath a new surface.

What to check next

  • Identify the membrane before repairing
  • Check seams beyond the visible leak
  • Follow the manufacturer's preparation method

When should you ask a roofer to inspect it?

Arrange a survey for recurring leaks, soft or deflected decking, persistent ponding, split details, blistering or an unknown build-up. Openings may be needed before anyone can responsibly confirm whether an overlay is suitable or the roof must be stripped.

Safety first: do not climb onto a roof without suitable access, training and protection. Keep clear of loose coverings, unstable ceilings and water near electrical fittings.

Technical content reviewJ & L Welch Roofing & Paving Services Ltd · Over 20 years of combined roofing experienceReviewed 28 August 2026 · How our answers are produced

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